Metal deposites in oxide coatings



Patented Jan. 15, 1935 1,988,012

METAL DEPOSITS IN OXIDE COATINGS Ralph Bryant Mason, New Kensington, Pa., as-

signor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application October 27, 1932 Serial No. 639,850

9 Claims. (Cl. 148-45) This invention relates to the production of decocell, using as an electrolyte a solution of an acid rative coatings on aluminum and aluminum alloy such as sulfuric acid, chromic acid or oxalic acid, surfaces and it is particularly concerned with a is particularly suitable, but satisfactory results method of producing on such surfaces decorative may also be obtained by numerous other methods,

coatings in which the color eifects are obtained such for example as by simple immersion of the I by the deposition of metal. aluminum article in a solution of sodium car- Numerous recently-developed methods of probonate and potassium dichromate or similar oxiducing coatings which consist substantially of dizing solution without the application of electrialuminum oxide on the surfaces of aluminum and cal energy. Anodic oxidation is to be preferred,

10 aluminum base alloys have extended the applicahowever, as harder, more adherent coatings 10 tion of these metals to various uses and to the possessed of better adsorbent properties are proproduction of various articles for which they were duced. Particularly good results may be obtained heretofore considered unsatisfactory. These soby making aluminum or aluminum base alloy arcalled oxide coatings, being dense and adherent, ticles to be coated the anode in an electrolytic protect the surfaces of the metal from corrosion cell, using as the electrolyte a solution of sulfuric l5 and, being more or less porous and adsorbent, lend acid. The conditions of temperature, time and themselves to the production of decorative effects current density used for the oxidation vary with on metal surfaces by the coloring of the oxide the concentration of the electrolyte used, and coating, when using sulfuric acid electrolytes, concentra- It is an object of this invention to provide a new tions of about 1 to 70 per cent acid may be used. 20 and improved method for the treatment of such A satisfactory adsorbent and adherent oxide coatoxide-coated aluminum and aluminum alloy suring may, for example, be obtained by anodic oxifaces to produce colored coatings having properdation of the aluminum article in a solution of 7 ties heretofore unknown in such coatings whereby per cent sulfuric acid bythe application of a curthe utility of the coatings is further increased and rent of 0.01 to 0.4 amperes per square inch of 25 they are made applicable to numerous new uses. anode surface for about minutes at room tem- It is more particularly an object of this invenperature. tion to provide a method for treating oxide-coated After a. suitable oxide coating has been formed luminum and aluminum base alloy surfaces to on the aluminum or aluminum alloy surface the 3 produce metallic deposits therein. coated metal is treated according to my invention 30 My invention is predicated upon the discovery to, produce a metallic deposit therein. The that if an aluminum or an aluminum alloy surface metallic deposit is Obtained y reati g the oxideprovided with an oxide coating be treated with coated metal successively with a solution of 2.

Solutions 61 one or more of certain class of reducible metallic salt and a solution or other metallic Salts and with a reducing agent t suitable form of reducing agent. The reducible metallic salt may be reduced in t and t metallic salts must be such salts as are soluble and deposit of free metal obtained in and on the oxide more 168$ reducible in lut o and they must coating. The metallic deposit thus produced be capable of complete reduction to the metal.

parts to the oxide coating a characteristic color The self-s of the metals of the oup o oup 1 '40 and it modifies the properties of t coating by of the Mendelejeif periodic system, copper, silver 40 increasing its corrosion resistance and by impartand gol Sometimes designated 35 the pp r ing to it the property of. conducting electricity, r up m a hav been f u d to p ss ss these although its resistance may be high. This comp p s n a h degree a d a parti ularly bination of properties has heretofore b adaptable to use in the coloring of oxide coatin s known in such coatings, as the oxide coatings according to my invention, but other suitab e themselves are generally considered to be excellent metal Salts y be used- The reducing a e t electrical insulators. used may be either in the form of a reducing solu- In carrying out my invention the oxide coatin tion or a reducing gas, or in any other adsorbable may be produced upon the aluminum r al miform, although it is usually preferable to use the 50 num alloy surfaceinany suitable manner whereby d ng agent in ution. The oxide-coated there is obtained a relatively hard, porous, adaluminum is treated with the solution of the reherent and adsorbent coating. For this purpose ducible metallic salt and with the reducing agent I have found that anodic oxidation of the alumiso that these reactants are adsorbed in and on the num or aluminum alloy surface in an electrolytic oxide coating whereby the reducing reaction is 55 produced within and on the oxide coating. The reducing action thus obtained deposits finely divided metal within and on the oxide coating.

The metallic deposits thus produced have characteristic colors which are useful for decorative purposes and which vary with the salts from which they are deposited and the method of their deposition. The metal deposits are usually in the state of division approaching colloidal size, and with variation in the size of the particles deposited variance in the shade of the characteristically-colored deposits is produced. For example, deposits of silver may be produced having the characteristic color of colloidal silver varying in shade from brown to brownish black. Various shades of reddish purple colloidal gold deposits may-also be obtained in this way, as well as yellow colored deposits of colloidal copper. The particular shades obtained vary with the salt of the metal first deposited, with the concentration of the solutions used, and with the particular reducing agent applied and its method of application.

In producing silver deposits the oxide coating may be first impregnated with a solution of substantially any soluble silver salt, but silver nitrate solutions have been found to be preferable. Particularly satisfactory for this purpose are alkaline solutions of silver nitrate in ammonium hydroxide, the salt in this form being readily reducible to the metal. The concentrationof the salt solution may vary between wide limits, depending upon the results desired, but it is generally preferable to use solutions containing about 1 to 20 per cent of the salt. The reduction of the silver salt is accomplished preferably by immersion of the impregnated oxide coating in a solution of a reducing agent. Solutions of reducing sugars, pyrogallic acid, hydroquinone, hydrazine sulfate, formaldehyde, tannic acid, or mixtures of these reducing agents may, for example, be used. The reduction may also be made by treatment of the solution-impregnated oxide coating with a reducing gas, such as by treatment with formaldehyde gas. The color of the metallic deposit obtained may be varied by reversing the order in which the solutions of reducible metallic salt and the reducing agent are applied to the oxide coating, and in using certain types of metallic salts it is preferable that the order of application of these solutions be reversed from that usually applied. For example, when a simple solution of silver nitrate in water is used it is usually preferable to treat the oxide-coated aluminum surface first with the solution of the reducing agent and subsequently with the solution of the reducible metallic salt, as better results are obtained thereby.

Copper deposits may be produced from solutions of salts similar to those used for depositing silver. Tannic acid and hydrazine sulfate solutions have been found particularly useful for use as reducing agents in conjunction with a solution of ammoniacal copper sulfate. Gold chloride solutions, preferably alkalinized with potassium carbonate, may alsobe used in conjunction with tannic acid, formaldehyde or other reducing agents to produce deposits of gold.

In the following examples, there are set out more specifically the results which may be obtained by my process, and the variations in color and shade resulting from varied conditions of operation.

Example I An aluminum sheet was first provided on its surface with an adherent and adsorbent oxide coating by subjecting it, as the anode, to the action of a 7 per cent sulfuric acid solution in an electrolytic cell, using a current density of 0.08 amperes per square inch of anode surface. An ammoniacal solution of copper sulfate was prepared by adding concentrated ammonium hydroxide to a solution of copper sulfate until the precipitate formed was just re-dissolved. This solution was diluted to a concentration of about 10 per cent. As a reducing solution, 0.5 grams of hydrazine sulfate was dissolved in 200 cc of water and about 0.5 per cent of caustic soda was added. The oxide-coated sample was immersed in this solution of ammoniacal copper sulfate and then, after rinsing, in the water solution of hydrazine sulfate. A yellow deposit of finely divided copper was thus obtained.

Example 2 A sample of aluminum sheet was first provided with an oxide coating as described above in Example 1. A solution of 1 per cent silver nitrate was prepared, to which a small amount of potasslum hydroxide was added. Sufiicient ammonia was then added to just re-dissolve the precipitate formed. A reducing solution of hydrazine sulfate was prepared as described in Example 1. The oxide-coated aluminum sample was first immersed in the ammoniacal silver nitrate solution, rinsed, and then immersed in a hydrazine sulfate solution. A brown deposit of colloidal silver was obtained.

Example 3 An oxide-coated aluminum article was first immersed in a solution containing 2.5 per cent pyrogallic acid and 2.5 per cent sodium thiosulfate or hypo. The article was then rinsed and immersed in a 10 per cent solution of silver nitrate. A very dark brown to black silver deposit was obtained.

Example 4 A reddish piu'ple color was obtained by depositing gold, in a colloidal or semi-colloidal state of division, in an oxide coating. In producing this color the aluminum article, provided on its surface with an oxide coating, was first impregnated with a 10 per cent solution of tannic acid. The impregnated coating, after rinsing, was then immersed in a solution of gold chloride containing 0.6 per cent of the salt, to which had been added 3 per cent of potassium carbonate.

The colored oxide coatings thus obtained are substantially permanent and are very suitable for decorating aluminum surfaces, particularly such surfaces as are to be used in exposed positions. In some cases, however, particularly where the lighter colored silver deposits are obtained, there is a tendency for such coatings to change color with exposure to sunlight. This effect may be due to the fact that in such cases a certain amount of unreduced or partially reduced metallic salt remaining in the coating is reduced by the action of the sunlight. The term "metal deposits as it has been used throughout this specification must, therefore, be understood to include not only deposits of completely reduced metal but also such mixtures of metal and unreduced or partially reduced metal salts as may be obtained by practicing my invention according to the above descriptlo Having now particularly described and pointed out my invention, what I claim is:

1. A method of coloring aluminum or aluminum alloy surfaces comprising producing on such surfaces an adherent and adsorbent oxide coatin and treating said coating successively with a solution of a reducible metallic salt and with a reducing agent.

2. A method of coloring adsorbent oxide-coated aluminum or aluminum alloy surfaces which comprises impregnating said oxide coating with a reducible metallic salt and treating said impregnated oxide coating with a reducing agent.

3. A method of coloring adsorbent oxide-coated aluminum and aluminum alloy surfaces which comprises depositing finely divided metal therein by treating said coating successively with a solution of a reducible metal salt and with a reducin agent.

4. A method of coloring adsorbent oxide-coated aluminum or aluminum alloy surfaces which comprises impregnating said surfaces with a solution of a soluble salt of a metal of the group consistin of copper, silver and gold and with a reducing agent.

5. A method of coloring adsorbent oxide-coated aluminum or aluminum alloy surfaces which comprises impregnating the oxide coating with a solution of an ammoniacal silver nitrate and treating said impregnated oxide coating with a reducing agent.

6. A method of coloring adsorbent oxide-coated aluminum or aluminum alloy surfaces which comprises impregnating said oxide coating with a solution of a reducing agent and subsequently treating said impregnated oxide coating with" a solution of a soluble silver salt.

'I. A method of coloring adsorbent oxide-coated aluminum or aluminum alloy surfaces which comprises treating said surfaces successively with a solution of a soluble copper salt and with a reducing agent.

8. A method of coloring adsorbent oxide-coated aluminum or aluminum alloy surfaces which comprises treating said coating successively with a solution of a soluble gold salt and with a reducing agent. 1

' 9. A method of coloring aluminum or aluminum alloy surfaces comprising producing on such surfaces an adherent and adsorbent oxide coating, and treating said coating with solutions of va reducible salt and a reducing agent capable of reducing said salt, by first absorbing material from one of said solutions in the coating and then treating with the other solution.

RALPH BRYANT MASON. 

